Headphones Amp

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ruffrecords

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Looking for an off the shelf headphones amp PCB with balanced inputs and powered by a single 12V rail that I can integrate into a mixer. Any suggestions?

Cheers

Ian
 

JohnRoberts

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12V supply sounds marginal (low) for an effective master section headphone amp. To accommodate the wide range of headphone impedances you could encounter. Good practice is generally to provide healthy signal swing to drive 600 ohm cans adequately but to also use a series resistance to current limit when driving low impedance phones.

A common newbie mistake when designing headphone amps into console master sections is not accounting for the significant headphone jack ground return current. Careless layouts can cause undesirable crosstalk from the headphone current corrupting clean signal grounds inside a busy master section.

Been there done that... decades ago.

JR
 

abbey road d enfer

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Bo Deadly

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LM386 link
No! LM386 is noisy.

Ah, the old headphone amp discussion. I suppose it's about that time again.

I agree with JR. The trick is how to handle different impedance loads. It seems to me just putting a buildout resistor inside the feedback loop of a typical amp would be a nice, and very simple, solution for that. Impedance is kept low for all loads and yet your not going to blow out your drums by accident.

I would just make a PCB though. There's always something not to like about whatever off-the-shelf module. And it should be so small that you can get some from OSHPark for less than whatever module.
 

sahib

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Ian,

I have some modules complete with volume potentiometer and jack output. I'll check tomorrow to see if they operate on single rail and e-mail you photos.
 

ruffrecords

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12V supply sounds marginal (low) for an effective master section headphone amp. To accommodate the wide range of headphone impedances you could encounter. Good practice is generally to provide healthy signal swing to drive 600 ohm cans adequately but to also use a series resistance to current limit when driving low impedance phones.
Interesting because the outputs will drive more than +20dBu into a 600 ohm load. Are you suggesting I can use this same output to drive lower impedance headphones simply by including a suitable series resistor?

Cheers

Ian
 

ruffrecords

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No! LM386 is noisy.

Ah, the old headphone amp discussion. I suppose it's about that time again.
I wouldn't normally ask because I would just use my regular tube one but in this one instance there is just no room so reluctantly I need a semiconductor answer
I agree with JR. The trick is how to handle different impedance loads. It seems to me just putting a buildout resistor inside the feedback loop of a typical amp would be a nice, and very simple, solution for that. Impedance is kept low for all loads and yet your not going to blow out your drums by accident.

I would just make a PCB though. There's always something not to like about whatever off-the-shelf module. And it should be so small that you can get some from OSHPark for less than whatever module.
I may well do that. I have so far come across designs using the LM386 (high distortion) and one using NE5532 with added output transistors. I was hoping this was a well trodden path and there would be well known readily available solutions. I know of a Doug Self design that uses a TL072 with a couple of output transistors, maybe that will do.

Cheers

Ian
 

abbey road d enfer

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Are you suggesting I can use this same output to drive lower impedance headphones simply by including a suitable series resistor?
It depends very much on your intention. If you want just something that indicates there is a decent signal, it's ok, but note that modern low-Z headphones are designed to operate wit a very low source Z. Adding a resistor in series changes significantly the bass response.
 

JohnRoberts

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Interesting because the outputs will drive more than +20dBu into a 600 ohm load. Are you suggesting I can use this same output to drive lower impedance headphones simply by including a suitable series resistor?

Cheers

Ian
Sounds like no problemo.... ;) The last I seriously researched headphone impedances I think I found an impedance range from as low as 3.2 ohm up to over 1k ohm... I suspect 600 ohm is a practical upper limit and maybe 8 Ohms on the low end. If you have higher end customers you can probably ASSume premium headphones (40-60 ohm). Maybe ask your customers. Adding a series resistance was common practice, not exactly audiophile.

===
The high minimum gain for old headphone amp chips is likely a stability concern. You can trick them into thinking they are operating at higher noise gain with an added RxC from - input to ground. Selecting a small enough cap can forgo any audible noise contribution.

There are probably newer chip sets that I am not aware of.

Have fun... I really don't miss juggling these sundry tradeoffs.

JR
 

ruffrecords

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I also came across a thing called the 47 amp which seems to be an old headwize project using both op amps of a NE5532 in a current boosted configuration. But I doubt this will drive a 32 ohm headphone.

Cheers

Ian
 

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JohnRoberts

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Headphone amps have been well inspected here over the years.

I suspect a search will find TMI from the regular suspects.

JR
 

Bo Deadly

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The simplest most straight forward design I think is to just parallel two op amps like the O2 amp described here:

O2 Details

See the Documentation Package for the schem. After much back and fourth NJM4556 proved to provide low noise, THD and good power.

D. Self also highly recommends the parallel op amps trick.

But again, I would put the feedback resistors on the INSIDE of the feedback loop. Lets say each one is 47R so two in parallel means the series resistance with the output is effectively 24R. This limits how much power you can dump into someones ear drums yet the impedance is very low which gives maximum fidelity. This is not true if you put the buildouts on the outside.

More specifically, imagine the amp output is at +4.5V (with a 12V supply in your case you have +-6 but the amp is only going to be able to swing within ~1.5V of each rail). So with the 24R in series and 8R load, when the output is 4.5V, the voltage across the load is 1.125V. Ip is 4.5 / (24 + 8) = 141mA * 1.125 = 159mWp. Repeat with a 600R load and it's 31mWp. So the 600R is only marginally lower power than what you can do without buildout resistors and +-4.5V but 159mWp with an 8R load is plenty loud. But, and this is the point of all of this, without the buildout resistors the 8R load will put out over 3 watts which could leave someone hobbled if they can't rip the phones off their head fast enough.

If you must have maximum power through 600R, then rearrange the dual amp circuit to make a differential out and put 9V across the load. Build out resistors inside the feedback loop still apply equally well.

Use a beefy rail-to-rail amp and squeeze every mW possible out of it.

However, there is another major gotcha. You can't DC couple the output with a single supply unless maybe the output is differential. Be careful not to torch the customers headphones! And watch those offsets. Especially on startup. Maybe put some of the buildout resistor on the outside with a 470uF 10V cap in front of it.

Finally, and this must be said, there are headphone amp ICs that have protections builtin. The fidelity is probably not up to most peoples standards but honestly I don't really care that much about THD as much as I do noise. At least not at the levels were talking about. The phones themselves probably add more distortion than a lowly LM386.
 
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Whoops

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People seem to praise the O2 headphone amp,
there's also another project that people seem to like although simpler it's the CMoy headphone amp.
Although both the O2 and CMoy use bipolar power supplies and not just 1 rail.

 

moamps

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